Electrophotographic printers having an LED array for the exposure light source have been employed as a printer terminal for personal computers and work stations. In the conventional electrophotographic printers, the LED array is comprised of LED chips each of which is made up of a plurality of LED elements formed on a single chip and arranged in a column, and which are disposed side by side in a line to provide the required dimension corresponding to the width of the recording paper.
Because the brightness may vary from one LED chip to another, it is necessary, in forming a line of LED chips, to select the chips having identical brightness. As a result, the yield of the chips is low. Moreover, the assembly requires much labor, resulting in a higher cost.
To improve this situation, electrophotographic serial printers employing a LED array comprising a single LED chip have been proposed (Japanese Patent Kokoku Publication No. 23033/1985, and Japanese Patent Kokoku Publication No. 23034/1985). The electrophotographic serial printers have a carriage that moves back and forth in a direction in which perpendicular to the direction of the recording paper is fed, and devices for the respective processes of the electrophotography (charging, exposure, development, transfer, fixing and cleaning) are mounted on the carriage. Magnetic toner images formed on the photosensitive member are transferred to the recording paper, utilizing a suitable magnetic source, and fixed by applying heat from a heat source to the toner on the recording paper.
Systems using electrostatic forces for the transfer to the recording paper have also been proposed (Japanese Patent Kokai Publication No. 152463/1986). Transfer of the toner to the recording paper is made for a certain number of printing lines and the recording paper with the toner image unfixed is advanced to the fixing means where the toner image is fixed.
With the electrophotographic printer utilizing the magnetic source for image transfer, the toner that is used must be a magnetic toner, which is more expensive than an ordinary toner which does not contain magnetic powder.
With the electrophotographic printer utilizing electrostatic forces, the toner after the transfer is attached to the recording paper only with a weak force, so when the carriage is returned to the original position, or when the next line is printed, the carriage is brought into contact with the unfixed toner, resulting in disturbances of the print output (printed toner images).
If an electrostatic force is utilized for the image transfer to the recording paper, and heat from the heating source is applied on the toner on the recording paper for fixing the image, toner other than magnetic toner may be used, and the toner image is not disturbed even if the toner an the preceding line and the photosensitive member are in contact with each other when the next line is printed. The heat efficiency in this non-contact fusing method however is very low, and the recording speed cannot be increased much. Moreover, the temperature in the device is increased, so that the recorded image is prevented from being stabilized. Furthermore, separate devices are required for the transfer process and the fixing process, so that the size of the entire device and its cost are increased.
A further problem associated with the prior-art electrophotographic printer is that recording is performed while the carriage is moved in one direction only and is not performed while the carriage is moved in the other direction. The recording speed is therefore limited.